In a line transfer type of CCD imager the image register comprises a plurality of parallelled CCD charge transfer channels in which charge packets representative of a radiant energy image are caused to form. The charge packets in each CCD channel represent a line of image response, and these lines of image response are sequentially transferred from the image register. Transfer may be to a common read-out bus connecting the output electrodes for each of the image registers. A disadvantage of using such a read-out bus is that the output signal-to-noise ratio is diminished because of the high capacitance of the long readout bus. Alternatively, the common read-out bus may be replaced by a CCD charge transfer channel running alongside the imager register, its successive stages arranged for side-loading from respective ones of parallelled charge transfer channels in the image register. The side-loaded CCD charge transfer channel introduces a time skew between the lines in excess of that associated with their being regularly scanned, so delay compensation must be made to avoid shear distortion in a television image reproduced from the imager output signal.
P. K. Weimer, the inventor, in U.S. Pat No. 3,811,055, issued May 14, 1974 and entitled "CHARGE TRANSFER FAN-IN CIRCUITRY" describes a way that charge packets from an array of successively read-out parallelled charge transfer channels may be multiplexed to supply a continuous serial flow of charge packets to an electrometer output stage, with each row of charge packets transferred from one of the charge transfer channels subjected to the same delay during read-out as every other row. The fan-in circuitry performing this process is a converging-tree structure of channel merging stages. The channel merging stages are arranged in progressively thinning ranks, with charge packets from adjoining charge transfer channels at the input of each successive rank of channel merging stages being routed to a reduced number of charge transfer channels at its output.
A 2:1 reduction in the number of charge transfer channels in each rank of channel merging stages is particularly described. This type of fan-in circuitry can be used, the present inventor indicates, in time-division multiplexing the charge packets supplied from the image register of a CCD imager of the line transfer type. Each successively selected line of charge packets read out of the image register is, then, routed to a single charge-sensing stage which converts each charge packet to a corresponding video output signal sample. (Note that each channel merging stage receives charge at any given time from one, not both, of the charge transfer channels at its input because of the selection of one row at a time in the imager for read-out.)
In U.S. Pat. No. 3,811,055, it is indicated that nine ranks of successive channel merging stages suffice to combine the output packets successively clocked from up to 2.sup.9 image register rows, a number larger than the 480 or so active lines in an NTSC television field. A problem that is recognized is that, as the number of rows of charge packets that are to be multiplexed increases (say, beyond four), the outputs of the channel merging stages in succeeding ranks become more widely separated on the semiconductor substrate, making it difficult to continue the successive channel merging further. To deal with this problem diffused islands in the substrate are proposed, to be used as connecting bars for conducting charge packets to the inputs of channel merging stages in the later ranks of the fan-in charge transfer structure.
The long diffused islands are not without associated problems, however. Considering the operation of these diffused islands as analog transmission lines, the length of the diffused islands makes it very difficult to complete the propagation of signal along them within one clocking interval. Accordingly, a CCD structure which would time-division-multiplex the signal charges from common output electrode with the same time delay for each register is desired, which structure avoids long interconnecting buses.
Channel merging stages can be used with time division multiplexing, as just described, or they can be used for merging the output charge packets simultaneously supplied from the output ends of a plurality of input charge transfer channels. In this latter type of channel merging stage, which is the type most often encountered in practice, the output charge packets are the sum of the input charge packets. Accordingly the charge transfer channel coming out of the stage has to accomodate the larger output charge packets. Customarily gate electrode lengths and clocking potential swings are maintained uniform through the channel merging stage. So the width of the gate electrode structures and the underlying charge transfer channel increase, and the the need for increased charge transfer capability is inherently accomodated.
The present inventor has discerned that, where channel merging is used in the time-division-multiplexing of sequentially supplied charge packet trains, the channel merging stage (contrary to the above-described practice) should be narrower at its output end than at its input end. Such a channel merging stage is to be referred to as a "charge funnel". In channel merging stages used in time-division multiplexing, the output charge packets are of the same order of size as the selected input charge packets. The channel merging stage comprises in addition to parallelled input charge transfer channels a further, output charge transfer channel, wide enough at its input to connect from the parallelled outputs of the input charge transfer channels, as in the channel merging stages for summing parallelly supplied trains of charge packets. Clocking potential swings are maintained uniform through the charge merging stage, per custom. But the output charge transfer channel is narrowed to be at its output end only about as wide as the output end(s) of the one or two input charge transfer channels supplying input charge packets at any given time. This narrowing is possible since there is no appreciable risk of overfilling the potential wells induced under the gate electrodes at the narrowed output end of the output charge transfer channel. There is no appreciable risk because charge packets are introduced into the output charge transfer channel from only one or two of the input charge transfer channels at a time.
In accordance with Coulomb's Law, the admission of input charge packets under the wide gate electrodes overspanning the wide input of the output charge transfer stage will, owing to the large capacitance of these gate electrodes, result in very small potential changes under these electrodes. Where charge is taken out of the output charge transfer channel through an end drain, this small potential change presents no problem. But where an electrometer (such as a floating-diffusion type) is used as an output stage, sensitivity of the output stage is adversely affected. Narrowing the width of the output charge transfer channel along with its overspanning gate electrode structures will, at the output end of the output charge transfer channel, again in accordance with Coulomb's Law, restore the sensitivity of potential change in response to input charge packets. So, the sensitivity of the electrometer stage measuring charge packets transferred to the end of the output charge transfer channel will be improved, owing to the narrowness of the charge transfer channel at its output end.
Signal-to-noise improvements are obtained with a narrowing channel merging stage before the electrometer output stage. The greater sensitivity of the electrometer increases output signal strength compared to the noise generated in the electrometer and associated output amplifier(s). But, further, the reduction in the combined areas of the gate electrodes overspanning the channel merging stage when it progressively narrows towards its output end reduces the accumulated dark current charge accompanying the charge sensed by the electrometer. So there is a reduction in the noise associated with variation in dark current charge, which can be a major factor adversely affecting signal-to-noise ratio.